1. Field of the Invention
The invention relates to an improved process for the production of acetic acid by methanol carbonylation.
2. The Related Art
An important process for the production of acetic acid is the carbonylation of methanol with carbon monoxide in a liquid reaction medium. The reaction is generally carried out in the presence of a catalyst, e.g., a Group VIII metal catalyst such as rhodium, a halogen containing catalyst promoter, e.g., methyl iodide, and water. An ionic catalyst stabilizer/co-promoter such as lithium iodide may also be present. A disadvantage of this process is that, in addition to the desired acetic acid, various amounts of undesirable impurities are also produced, e.g. permanganate reducing compounds (PRC""s) including carbonyl compounds such as acetaldehyde, unsaturated aldehydes such as crotonaldehyde and 2-ethylcrotonaldehyde, saturated aldehydes other than acetaldehyde such as butyraldehyde and 2-ethylbutyraldehyde, and ketones such as acetone and methyl ethyl ketone, all of which are involved in the determination of xe2x80x9cpermanganate time,xe2x80x9d an important commercial test which the acetic acid product must pass for many end uses. Other undesirable impurities produced in the methanol carbonylation process are propionic acid which reacts similarly to acetic acid in the production of products such as vinyl acetate but often has an adverse effect on the properties of such products, and when methyl iodide is used as a catalyst promoter, higher alkyl iodides, e.g., C2-C12 alkyl iodides such as ethyl iodide and hexyl iodide which are more difficult to remove from acetic acid by distillation than methyl iodide and which among other adverse effects, poison the catalysts generally used in the production of vinyl acetate from acetic acid.
It is believed that, in addition to reducing the selectivity to acetic acid of the methanol carbonylation process, the acetaldehyde formed in the reaction is instrumental in the formation of several of the foregoing impurities by undergoing further reaction. Thus, acetaldehyde under typical reaction conditions may form various hydroxyaldehydes by aldol condensation and cross-aldol condensation, which are then dehydrated to form unsaturated aldehydes such as crotonaldehyde and 2-ethylcrotonaldehyde. Moreover, because of the presence of hydrogen in the reaction section due to the water gas shift reaction, some of the acetaldehyde may be reduced to ethanol, which is then carbonylated to propionic acid. Finally, when methyl iodide is present as a catalyst promoter and/or an ionic iodide such as lithium iodide is used as a catalyst stabilizer/co-promoter, such compound may react with acetaldehyde or a carbonyl compound synthesized from acetaldehyde to form one or more C2-C12 alkyl iodides which are more difficult to separate from acetic acid than is methyl iodide.
In view of the reduction of selectivity to acetic acid caused by the formation of acetaldehyde and the adverse effects resulting from the presence in the acetic acid product of even small amounts of acetaldehyde and compounds formed from acetaldehyde both during the methanol carbonylation reaction, and the purification of the acetic acid product, a method for reducing the amount of acetaldehyde in any part of the reaction and/or purification sections of the system is desirable.
U.S. Pat. No. 3,769,329 discloses a process for the production of acetic acid by the carbonylation of methanol with carbon monoxide in the presence of a catalyst system comprising rhodium and a halogen component wherein the halogen is iodine or bromine.
U.S. Pat. Nos. 5,001,259; 5,026,908; and 5,144,068 disclose processes for the production of acetic acid by the carbonylation of methanol with carbon monoxide in a liquid medium containing a rhodium catalyst stabilized with an iodide salt along with an alkyl iodide, an alkyl acetate and an unusually low concentration of water.
Various methods for reducing impurity levels in acetic acid production processes have been disclosed. U.S. Pat. Nos. 5,155,265, 5,155,266, and 5,202,481 disclose the purification of acetic acid made by low water carbonylation of methanol with carbon monoxide and containing iodide, unsaturates and carbonyl impurities, by treating the final product with ozone.
U.S. Pat. No. 5,625,095 discloses a process for the production of acetic acid by the carbonylation of methanol with carbon monoxide in the presence of a rhodium catalyst, iodide salts and methyl iodide wherein the acetaldehyde concentration in the reaction liquid is maintained at 400 ppm or lower by contacting the liquid containing the carbonyl impurities with water to separate and remove such impurities.
M. Gauss, A. Seidel, G. P. Torrence and P. Heymann""s, xe2x80x9cSynthesis of Acetic Acid and Acetic Acid Anhydride from Methanolxe2x80x9d in B. Cornils and W. A. Herrmann, Applied Homogeneous Catalysis with Oryanometallic Compounds, Vol. 1, p. 104-138, (New York: VCH, 1996), is a survey description of the process of synthesizing acetic acid and acetic anhydride by the carbonylation of methanol and methyl acetate respectively with carbon monoxide using metal complexes such as those of rhodium and cobalt as catalyst and a methyl halide such as methyl iodide as promoter. FIG. 1 on page 114 of volume 1, is a flow diagram showing a process for the production of acetic acid using a rhodium catalyst. The reactor liquid is passed to a flasher where acetic acid product and a majority of the light ends components are separated as vapor from the catalyst solution and are then fed to a light ends distillation column where such light ends components are further separated as overhead from the bulk of the acetic acid product, such overhead being condensed into two phases, one being predominantly aqueous and the other predominately organic.
U.S. Pat. No. 6,143,930 discloses processes for the synthesis of acetic acid by carbonylation of methanol, including the step of removing the acetaldehyde in the system by twice distilling the aqueous phase, i.e., the lighter liquid phase of the overhead from the light ends distillation column. Optionally, the overhead from the last column may be subjected to an aqueous extraction to recover residual methyl iodide for recycle before sending the extract containing acetaldehyde to waste treatment.
All patents and publications referred to herein are hereby incorporated herein by reference in their entireties.
The present invention relates to a process and a system for producing acetic acid by the catalytic carbonylation of methanol with carbon monoxide to obtain a reaction product stream comprising acetic acid and a minor amount of acetaldehyde. The acetaldehyde in the reaction product stream is partially converted by oxidation to acetic acid or further to CO2 and H2O. The stream may then be directed to the purification section, the reaction section, or both, thereby reducing the deleterious effects of acetaldehyde. In conventional systems, acetaldehyde is isolated for removal as a waste stream component. One of the advantages of the present invention over conventional processes is the elimination of at least one waste stream or reducing the quantity of waste produced. Additionally, the conversion of acetaldehyde to acetic acid through the oxidation process increases the efficiency of the overall process.